This invention concerns cutting inserts for the metalworking industry and is especially concerned with an indexable cemented carbide cutting insert used with toolholders for machining metal workpieces.
Cutting insert designs and configurations are many in the metalworking art. Some of the more recent configurations may be seen by a review of the McCreery et al U.S. Pat. No. 3,973,307. This patent disclosed a new style cutting insert that recognized that chip control could be effected without the use of molded chipbreaker grooves or superstructures that would impede the flow of the chip and increase the horsepower requirements.
In the industry of metalworking, chip control is very important. When the chip is separating from the workpiece, it is very desirous to have it curl first and then break off from its parent chip so that discrete, individual, curled chips provide a safer environment for the machine operator and, furthermore, enhance safer handling of the waste chips when being further processed or handled in the machine shop or elsewhere.
When referring to the curled chips, the terms tight and loose are used. Tight refers to the condition where the chip is tending to curl too much and chips may break off in a smaller size than would be desired. On the other hand, loose chips are chips that are not curling as much as the ideal chip and may be breaking off in larger size chips than would be desired. Within certain ranges of tight chips and loose chips, the chips may be acceptable in metalworking production, unless a chip is referred to as too tight and too loose, in which case the chips are then unacceptable.
The tightness of the curled chip affects power consumption and the rate of wear of the cutting insert. As the tightness of the curled chip increases, the pressure with which the chip contacts the insert increases, thereby producing increased cratering of the insert in the area of contact. This situation leads to increased power consumption and reduced insert life time.
Prior insert designs, such as McCreery et al, have consisted of a polygonal body having top and bottom faces joined by a peripheral wall. Formed at the juncture of one face wih the peripheral wall are two adjoining and angular related cutting edges forming a corner. Extending inwardly from the cutting edges is a land area. At the inner edge of the land area is a descending wall which joins a horizontal face which extends toward the center of the insert.
Inserts of this general class have been found to be efficient and have good chip control over a range of speeds, feeds and types of metal workpieces. However, when these inserts are used on long continuous machining operations on a single workpiece, there has been a tendency for the cutting edge around the corner or nose area of the insert to break off. This tendency toward early failure of the corner area in these applications is believed to be due to an increased rate of cratering occurring on the angularly related land and descending wall near the corner and behind the edge doing the cutting. A possible mechanism, explaining the cause of increased cratering, is as follows: As a chip is formed at the cutting edge, it slides down the descending wall, contacts the horizontal face and is caused to curl. However, in the nose or corner area, where the cutting edge land and descending wall join the adjacent and angularly related land and descending wall, which for clarity are now called the side land and side descending wall, a problem arises.
The side land and side descending wall interfere with the free flow of the chip down the nearby cutting edge descending wall. That end of the chip contacting the side land and side descending wall takes on a tighter curl than the rest of the chip away from the corner area. In the area of contact between the chip and the side land and side descending wall, higher contact pressures are produced than are produced along the cutting edge descending wall. This causes an increased rate of cratering in this contact area thereby weakening the corner area and producing the observed tendency for the corner area to be the first portion of the insert to fail during long machining runs.
This tendency for early failures in the corner area of the insert during long machining runs has not been alleviated by the addition of a secondary land or step behind the descending wall as illustrated in Mundy U.S. Pat. No. 4,087,193.
It should also be noted that, while bumps have been placed on inserts in the past (see, for example, U.S. Pat. No. 4,214,845), they have not been used in conjunction with a secondary land to achieve a reduction in the rate of crater wear on the side land and descending wall. Their primary use has been as chip groovers and chip breakers.